A variety of instruments and methods have been used to measure geophysical phenomena. Pressure sensors and seismic instruments are used in disaster warning systems as described in the referenced documents “Nano-Resolution Sensors For Disaster Warning Systems” and “Nano-resolution technology demonstrates promise for improved local tsunami warnings on the MARS project”. Seismometers, gravimeters and tiltmeters are extensively used for geodetic measurements. Applications in energy exploration and production include instruments and methods for seismic refraction measurements, orientation modules for directional drilling, and characterizing hydrocarbon reservoirs through gravity measurements. Tilt measurements are useful for providing geodetic data on subsidence and uplift, whether due to natural earth movements or due to energy production. Hydrocarbon reservoirs may be characterized with surface and borehole gravity measurements.
Prior art devices generally use the direction of gravity as a vertical reference or “plumb line”. Compass directions may be determined with devices oriented to Earth's magnetic poles. Gravimeters and vertical seismometers can be aligned to the plumb line to measure gravity and tiltmeters can measure the angular deflections of the plane perpendicular to the plumb line. Various techniques in the prior art are used to ensure alignment and/or characterize misalignments to the plumb line. These techniques include leveling, dynamic compensation and characterization using associated sensors such as tiltmeters.
The referenced document, “Lacoste and Romberg Stabilized Shipboard Gravity Meter”, and U.S. Pat. No. 3,474,672 describe a leveling platform that maintains verticality between the gravity meter and the plumb line using gimbal mounting and servo loops. Leveling techniques are also used to maintain verticality of relative gravity meters as described in U.S. Pat. Nos. 4,445,371, 4,457,077, and 4,457,168. U.S. Pat. No. 5,461,914 describes an absolute gravity meter that also requires leveling. Gyroscopes are used to determine the gravity meter's spatial angle from verticality as described in U.S. Pat. No. 5,112,126. Tiltmeters and accelerometers are used in a servo leveling system to align to verticality as described in U.S. Pat. No. 5,970,787. U.S. Pat. No. 8,616,054 describes a seismic and gravity sensor in which the weight of an inertial mass is unloaded with a spring arrangement when exposed to, and aligned with, the force of the static gravity field.
U.S. Pat. No. 3,630,086 describes a dynamic compensation technique wherein differential gravity forces are balanced by induced centrifugal forces. U.S. Pat. No. 5,341,681 describes a dynamic compensation technique using rotating accelerometers to determine the in-line and cross-gravity components in a gradiometer. Dynamically rotating a tiltmeter and accelerometer assembly about the instrument axis can be used to determine the cross terms in an alignment matrix as described in U.S. Pat. No. 5,606,124. U.S. Pat. No. 5,728,935 describes a system of two force measuring assemblies and an independent measure of kinematic acceleration from a reference point such that differential measurements of gravity may be calculated. U.S. Pat. No. 6,518,756 describes an apparatus for determining the motion of a borehole tool from rotational dynamic measurements relative to the plumb line using accelerometers and magnetic induction sensors. U.S. Pat. No. 6,590,536 describes a body motion detection system that corrects body-mounted accelerometers for the effects of static gravity. Another dynamic compensation technique is described in U.S. Pat. No. 6,615,660 wherein the perturbations caused by the motion of a moving mass are used to compensate and correct the outputs of a gradiometer. U.S. Pat. No. 8,789,415 describes a gravity gradient measuring instrument using two dynamically rotating sensor masses to reduce the effects of common-mode aircraft accelerations.
U.S. Pat. No. 6,776,035 describes a subsurface gravity measuring device in which a gravity sensor and separate tiltmeters are calibrated initially with characterization coefficients and later rotated relative to the plumb line to derive recalibrated coefficients. Because the gravity sensor and tiltmeters have full scale ranges less than 1 G, the rotation angles used for recalibration are small and a rotationally invariant 1 G vector cannot be used as a reference.
There are many single axis accelerometers in the prior art that can be combined into a triaxial acceleration assembly. Inherently digital sensors that include vibratory structures whose resonant frequencies are functions of the applied accelerations are described in U.S. Pat. Nos. 4,467,651 and 4,479,385. A triaxial accelerometer with variable sensitivity is described in U.S. Pat. No. 7,178,401. U.S. Pat. No. 6,826,960 describes a triaxial acceleration sensor with an internal alignment matrix.
Traditional strong motion sensors do not have the sensitivity or stability to make good long-term geodetic measurements. Traditional broadband seismometers and tiltmeters operate over a small fraction of Earth's 1 G gravity vector and do not have the range to measure strong seismic events and have no absolute reference for long-term measurements. Thus there is a need for a device and in-situ calibration method for improved seismic and geodetic measurements.